The present invention relates to a method and device for detecting and correcting modulation errors of an information-carrying modulated signal.
For the transfer of digital signals over longer distances between a transmitting unit and a receiving unit, it is suitable to code the digital signal in some way. The coding is done in order to ensure a reliable information transfer despite interference. For transfer by wire, one of the easiest forms of coding can be to increase the signal levels and a more advanced form can be some variation of phase shift or frequency shift of a carrier wave. The use of optical fibres for information transfer demands the use of a light source which is coded by the digital signals.
Transfer of information over wires or optical fibres is not always possible due to geographical positions, costs or other circumstances. In such cases it can be more suitable to use wireless transfer of the information, for example with the aid of some type of radio waves. Even in this case it is suitable to make use of a carrier wave which is modulated by the digital information.
The three basic methods of modulating a carrier wave with the aid of digital information are by means of a modification, a step, of either the carrier wave's amplitude, frequency or phase. The three basic methods of modulating a carrier wave are normally called amplitude shift modulation, frequency shift modulation and phase shift modulation respectively. Which method is used can be based on any of the following desires and/or requirements: detection performance, data speed, available spectrum/bandwidth, hardware complexity, frequency range, cost etc. Some of these requirements/desires conflict directly with one another, for which reason a prioritising dependent on the application has to occur. With amplitude shift modulation, the carrier wave's amplitude will vary, i.e. amplitude shift modulation does not have a constant envelope. Frequency and phase modulation both have a constant envelope which makes them insensitive to amplitude non-linearities which can occur in the transfer between a transmitter and a receiver. Examples of where amplitude non-linearities can occur are with the use of microwave radio links and satellite channels. Consequently, frequency and phase shift modulation are much more common than amplitude shift modulation.
Frequency shift modulation is most often the easiest to implement practically, i.e. the necessary hardware is simple both on the transmission side and on the receiving side, which gives low costs. Phase shift modulation on the other hand gives a better system in terms of performance, but requires a more complicated transmitter and receiver. Due to these differences, development has produced a type of hybrids which uses advantages from both modulation methods. Amongst these can be mentioned TFM (Tamed Frequency Modulation) and C-QPSK (Constant envelope offset Quadrature Phase Shift Key) which are basically the same modulation method, but which by virtue of the different names highlights the fact that it concerns a hybrid solution. With use of the modulation method, a physically simple transmitter is used similar to that used with frequency shift modulation and an advanced receiver similar to that used with phase shift modulation. In this way a system is obtained with advantages from frequency shift modulation, simpler and cheaper, and phase shift modulation, better performance.
However, certain problems arise when using modulation methods where the transmitter signal is generated by controlling a voltage-controlled oscillator (VCO) and where the signal is phase demodulated in the receiver. If a voltage-controlled oscillator is used to generate a modulated transmitter signal which is phase-modulated in a receiver, the requirement on the frequencies which the voltage-controlled oscillator generates are particularly high if a correct phase modulation is to be able to occur in the receiver. The requirements increase additionally with applications where the phase information over a number of information bits is used to generate a carrier wave frequency in the receiver, so-called coherent phase demodulation. The phase error is the same as the time integral of the frequency error.
With the generation of the transmitting signal's frequencies, a pulse shaper is used which converts the information-carrying signal, often a digital bit stream, into a modulating base band signal which controls the voltage-controlled oscillator. The level of the modulating base band signal defines which frequency the transmitter signal should have for a particular occasion. If the pulse shaper has an amplification error, which can be caused by temperature variations, aging of the components, bad initial adjustment of levels etc., this means that the voltage-controlled oscillator receives an incorrect input signal and as a consequence will generate incorrect frequencies. Due to amplification errors, the frequencies which are generated are scaled with a factor which is in relation to the amplification error. The scaling means that the width of the frequency spectrum which all the generated frequencies produce will reduce or increase to a degree which corresponds to the scaling.
The scaling can be illustrated in the following way: assume that the desired modulating base band signal has a range which lies between four and eight volts. If the amplification error gives rise to a factor of two, the input signal to the voltage controlled oscillator will lie between eight and sixteen volts. The output control range has thus not only moved, but also increased from four to eight volts and, in a similar way, the output signal's frequencies have thus changed. All of the frequencies have thus not increased the same amount, but instead the frequencies are scaled in relation to the amplification error. This creates an erroneous deviation of the output signal. By deviation is meant the instantaneous variation in the frequency from a carrier wave's envisaged centre frequency.
Problems with the frequency generation can also arise in the voltage-controlled oscillator, i.e. even in those cases where the modulating base band signal is correct, the voltage-controlled oscillator can generate the wrong frequency. The voltage-controlled oscillator can be marred with amplification errors which can be caused by temperature variations, aging of components, bad initial adjustment of levels and amplification factors etc. Here also, the generated frequencies will be scaled with a factor which is in relation to the amplification error.
In order to solve problems with generating correct frequencies, the transmitter can be provided with a deviation detector which measures the deviation of the modulated transmitter signal and thereby adjust the level of the base band signal in such a way that correct frequencies and thereby also correct modulation is obtained. In order to realise this, the transmitter is provided with a demodulator which demodulates the transmitter signal from the voltage controlled oscillator. The demodulated transmitter signal is then transferred to a deviation detector which detects the deviation and calculates the size of the amplification error and gives a deviation error constant. The deviation error constant is then used for correction of the level of the base band signal.
One problem that arises is how the deviation and the deviation error detection itself can be carried out in a reliable and simple manner. A large problem with deviation detection is when there is no indication about what the phase changes should be. A transmitter without a deviation detector can for example need a restructuring/upgrade with a deviation detector. A transmitter which previously had no deviation detector can be built up and integrated in such a way that it is not possible with a reasonable amount of work to obtain a signal with the digital information which is transferred via the transmitter, i.e. the digital information which is actually transmitted by the transmitter and which is not necessarily the same as the digital information which is supplied to the transmitter. If there is no possibility to obtain a signal with the digital information which is transferred into the transmitter, there is also no indication for supplying to a deviation detector concerning what the phase changes which are sent should be.
The deviation has to be able to be measured and corrected with sufficiently high accuracy in order, for example, that a coherent phase demodulation which is normally used is able to be performed.
One method of detecting the deviation and calculating the deviation error is to calculate the received phase changes per symbol and thereafter threshold-detect for a determination of the symbol's nominal phase change assignation. One symbol is the amount of information, one or more data bits, which a single phase change represents. This means that if the phase change per symbol is nominally -90.degree.(-.pi./2), -45.degree.(-.pi./4), 0.degree., 45.degree.(.pi./4) or 90.degree.(.pi./2), it can be suitable to use -67.5.degree., -22.5.degree., 22.5.degree. and 67.5.degree. as threshold levels for determination of the phase change assignation. If the detected phase change is larger than 67.5.degree. the nominal phase change is assumed to be 90.degree.. If the detected phase change lies between 22.5.degree. and 67.520 the nominal phase change is assumed to be 45.degree.. If the detected phase change lies between -22.5.degree. and 22.5.degree. the nominal phase change is assumed to be 0.degree. and so on. This is valid on the condition that no, or only a small, centre frequency error exists.
The method described above for detecting deviation errors has at least two essential problems, namely that it has a limited detection range and that the deviation detection is filter-dependent. If the deviation error is large, the threshold detection and therewith the deviation detection becomes erroneous. If the deviation error, which can be seen as a factorial error or scaling error, is less than 0.75 (0.75.multidot.90.degree.=67.5.degree.) or larger than 1.5 (1.5.multidot.45.degree.=67.5.degree.) the determination of phase assignation will be incorrect with threshold detection. The filter dependence arises due to the fact that transmitters and receivers normally comprise signal-adapted filters for optimising the signal/noise ratio and these filters affect, to a high degree, the detected phase change per symbol due to intersymbol interference. The effect is not symmetrical since it depends on the previous symbols and on the filters having low-pass characteristics. With a random sequence of data, the mean value of the detected phase changes is therefore reduced relative to a nominal theoretical mean value. This means that since the intersymbol interference is filter-dependent, the deviation detection also has an undesirable filter-dependence.